The magnetic recording medium for high density recording has been developed in recent years, and in that medium, the surface properties of the magnetic layer are required to improve more and more in order to reduce a so-called spacing loss between a magnetic head and a magnetic tape. For the above purpose, it is not only necessary to improve the surface properties of the magnetic layer by improving technologies for manufacturing a magnetic layer such as dispersing technology, coating technology or surface molding technology, but also the surface properties of the non-magnetic support. As the recording density increases, recording wavelengths decrease. Since an output level decreases (a so-called "thickness loss") as the recording wavelength decreases, the thickness of a magnetic layer is necessarily reduced so as to obviate the thickness loss. For the above reason, the surface properties of a non-magnetic support greatly influence those of a magnetic layer.
A magnetic recording medium having a thin ferromagnetic metal film comprised of Co-Ni, Co-Cr or Fe-N prepared by vapour deposition or sputtering has recently been developed to realize higher density recording. As the magnetic layer of the thin metal film type has much more reduced thickness than that of the coated type, the above described problem becomes more serious.
However, improving the surface properties of a support used in a magnetic recording medium results in some disadvantages to the recording medium. For example, when surface properties of a thus formed film are good, friction of the film against the roll for conveying the film increases at the step of winding up the film, whereby the film meanders or wrinkles. In some cases the shape of the rolled film becomes distorted due to the increased friction between films.
Many attempts have been made to solve the above-described problem. For example, a method which comprises extruding fine particles of thermoplastic resins on a support, dissolving and removing the particles by a solvent and thereafter forming a magnetic layer on the support is disclosed in Japanese Patent Application (OPI) No. 109605/78. The term "OPI" as used herein means an unexamined published application.
A method which comprises coating a solution of a polymer such as polyamide or polyester on a support, drying and forming wrinkles, and forming a magnetic layer on the support, is disclosed in Japanese Patent Publication No. 14555/71. Polyester and the like are used as a polymer to be coated on a support as described in Japanese Patent Publication No. 6117/72 or a thermoplastic polyester and the like are used to form fine wrinkles on a support as described on a support in Japanese Patent Publication No. 14555/71 and a magnetic layer is formed thereon, as disclosed in Japanese Patent Publication No. 38001/75.
However, the above-described four methods have been unsuccessful in providing a magnetic recording medium satisfactory for high density recording.
In this regard, a magnetic recording medium which is prepared by providing a non-magnetic layer containing a radiation-polymerizable compound and fine particles on a support, irradiating the layer ultraviolet rays and thereafter providing a magnetic layer thereon is disclosed in U.S. Pat. No. 4,619,856.
In accordance with the above technique, the surface properties and durability of the magnetic layer are relatively improved. But running durability is unsatisfactory and there is also a problem of heat resistance of a support provided with an under-coated non-magnetic layer. Particularly, when a thin metal magnetic film is to be formed on both surfaces of a support by vapour deposition or sputtering, after an under-coated layer is provided on both surfaces of said support and upon providing a magnetic layer on one surface by vapour deposition or sputtering, the under-coated layer on the back surface of the support has a tendency to stick to a surface of a substrate or a can. The sticking problem also takes place when a magnetic layer is provided by coating on both surfaces. That is, when a magnetic layer is subjected to a calendering treatment, an under-coated non-magnetic layer on the back surface of a support sticks to the roll for calendering.